01 - Set a heatproof bowl over a saucepan of gently simmering water (bain-marie). Add the chopped dark chocolate and unsalted butter, stirring occasionally until completely smooth. Remove from heat and allow to cool slightly for about 5 minutes.
02 - In a spotlessly clean bowl, beat the egg whites with a pinch of salt using an electric mixer until soft peaks begin to form. Gradually sprinkle in half the sugar while continuing to beat until stiff, glossy peaks hold their shape.
03 - In a separate bowl, whisk the egg yolks with the remaining sugar vigorously until the mixture turns pale yellow and falls in thick ribbons from the whisk.
04 - Pour the slightly cooled chocolate mixture into the beaten egg yolks and fold gently with a spatula until fully incorporated and no streaks remain.
05 - In a chilled bowl, whip the cold heavy cream with an electric mixer or hand whisk until soft peaks form — do not overbeat or the cream will become grainy.
06 - Gently fold the whipped cream into the chocolate-egg yolk base using a large spatula, working in smooth, sweeping motions to preserve the airy texture.
07 - Carefully fold the stiff egg whites into the mousse in three separate additions, using a light hand to avoid deflating the mixture. Each addition should be nearly combined before adding the next.
08 - Spoon or pipe the finished mousse into individual serving glasses or ramekins. Cover lightly with plastic wrap and refrigerate for at least 2 hours, or until fully set and chilled through.